Duplex driving mechanism



June 19, 1962 J. K. ROYLE ETAL DUPLEX DRIVING MECHANISM 5 Sheets-Sheet 1 Filed Sept. 28, 1959 INVENTORS JOSEPH KENNETH ROYLE BRIAN DAVID NELLIST June 19, 1962 ROYLE ETAL 3,039,579

' DUPLEX DRIVING MECHANISM Filed Sept. 28, 1959 5 Sheets-Sheet 2 n a 7 E 151 I 2 I H 251 I I 1 -I I FIG.2C. 'A] I 74/ 602 1; I W I l W/vE/V opmrm l 5%? I AU l r I F IG. 2D.

9 MON/70R I I I M! J -4 I I smvo I I 1 SIGNAL I I 1; COMP? I I G 2E I I L I mvm'rons JOSEPH KENNETH ROYLE BRIAN DAVID NELLIST BY 52:01 W

June 19, 1962 J. K. ROYLE ETAL DUPLEX DRIVING MECHANISM 5 Sheets-Sheet 3 Filed Sept. 28, 1959 JOSEPH KENNETH ROYLE jam DAVID N ST manc w v June 19, 1962 J. K. ROYLE ETAL 3,039,579

DUPLEX DRIVING MECHANISM INVENTORS JOSEPH KENNETH ROYLE BRIAN DAVID NELLIST BY w United States Patent f 3,039,579 DUPLEX DRIVHNG MECHANISM Joseph Kenneth Royle, Heaton Moor, Stoekport, and

Brian David Nellist, Manchester, England, assignors to National Research Development Corporation, London, England, a British corporation Filed Sept. 28, 1959, Ser. No. 842,815 Claims priority, application Great Britain Oct. 2, 1958 17 Claims. (Cl. 192-143) This invention relates to apparatus for driving a first object along a predetermined path in relation to a second object and is particularly applicable to the propulsion of the slides of machine tools along their slideways though the invention is not confined to such applications.

According to the invention there is provided driving apparatus for causing relative movement along a predetermined path between a first object and a second object, comprising a first driving mechanism and a second driving mechanism (hereinafter respectively called the first drive and the second drive) each such mechanism in turn comprising two members between which relative movement takes place to provide the driving action of the mechanism, the first of such members being secured to the first object and the second of such members being provided with means (hereinafter called the locking means) for releasably securing it to the second, object at different positions along -a track parallel to the said predetermined path, means (hereinafter called the actuating means) adapted in response to an incoming driving signal to initiate driving action on the part of the driving mechanism means (hereinafter called the positioning means) for positioning the first member relative to the second member when the driving mechanism in question is not executing a driving action and means (hereinafter called the indicating means) for indicating the relative positions of the two members at least at a plurality of predetermined relative positions of the two members, the apparatus further comprising means for operating the locking means of the first drive and releasing the locking means of the second drive, means for applying driving signals to the first driving actuating means, means for actuating the second drive positioning means to place the members of the second drive in predetermined relative positions, means under control of the first drive indicating means for operating the second drive locking means and bringing the second drive actuating means under control of the driving signals and means, operable when the locking means of both drives are operated and before either of the driving mechanisms has reached an end of its range of driving action, for releasing the locking means of that one of the two driving mechanisms which is nearest to an end of its driving range which it is at that instant approaching in the course of its driving action and removing the actuating means of that driving mechanism from the control of the driving signals, the first drive and the second drive being interchangeable in respect of the func tions of the various means further comprised in the apparatus as aforesaid whereby the two driving mechanisms may operate in turn to produce relative movement between the first object and the second object.

The invention will be more readily understood from the following description of certain. embodiments thereof illustrated in the accompanying drawings in which the first and second objects are respectively a work table and the bed of a machine tool and in which the driving mechanisms take the form of hydraulic piston cylinder units. The elements of the first drive are all given the suifix I and the elements of the second drive the suiiix ll.

FIGURE 1 is a diagrammatic representation of the 3,039,579 Patented June 19, 1962 FIGURES 2A, 2B, 2C, 2D and 2E are component portions of the circuit for controlling change-over between the two driving mechanisms.

FIGURE 3 shows the foot locking arrangement in cross section on an enlarged scale. 7

FIGURE 4 shows an embodiment of the invention which is a modification of the embodiment illustrated in FIG. 1 in that only one main hydraulic control valve is required.

FIGURE 5 shows a modification of the circuitry shown in FIGURES 2B, 2C, 2D and 2E.

FIGURE 6 shows an enlarged detailed arrangement of the locking device.

FIGURE 7 is a circuit modification.

A work table 1 is mounted on a slideway (not shown) carried by a bed 2. Both drives are similarly mounted so that the mounting of the first drive only need be described. Cylinder CI is fast with the work table 1. Piston PI rides in CI and is fast with the piston rod emerging from both ends of CI through glands in the cylinder ends. The outer ends of this piston rod are secured to brackets upstanding from a foot FI which rides on a slideway integral with the machine bed 2 and parallel to the slideway carrying table 1. Means described below in relation to FIGURE 3 are provided for releasably locking foot F1 to the slideway. Fll of the second drive rides along the same slideway as FI and the second drive is displaced from the first drive along the direction of movement of table 1 along its said slideways on bed 2, which may be the same slideways as those along which FI and F11 ride.

The cylinders CI and C11 have associated hydraulic valves VI and VII actuated by actuators AI and All respectively.

One of the drives is required to drive the table whilst the foot of the other is unlocked and when this drive approaches the end of its stroke its place is taken by the other drive which has in the meantime had its piston positioned so as to have a substantial part of its stroke avail able for driving action in the direction in which the work table is for the moment travelling. The changeover between the two drives involves the locking of the foot of the drive which is about to take over the driving function and the unlocking of the foot of the drive which is relinquishing the driving function. The driving signals must also be transferred from the actuator of the latter drive to that of the former drive.

In practice it is necessary that there should be an interval when for a short time both drives are participating in the driving action with their feed looked, so that there shall not be a period at the time of change-over when both the feet are unlocked since this would leave the table out of control.

Suppose that the first drive is operating to move table 1 to the right and the second drive is idling.

Piston PI will be moving towards the left hand end of cylinder CI. If the second drive is to take over the driving function for the same direction of movement of table 1, then its piston PII must be positioned away from the left hand end of its cylinder CII. This positioning may be done by suitable actuation of valve VII whose actuator All will not be connected to receive the driving signals. It is preferred however to provide independent positioning means for the drives and this necessitates removing each of them from the control of its valve. To this end taps TI and I'll are provided which can be operated to interconnect the opposite ends of cylinder CI and cylinder 011 respectively, the hydraulic pressure being at the same time cut off from the valves VI and VII or the valves being dissociated from their cylinders in any other convenient way.

' As, at the moment under consideration, the second drive is being positioned, T11 is open and the positioning means may be arranged to move PII a substantial part of the way towards the right hand end of its stroke.

Indicating means associated with the first drive now indicate that PI is approaching the left hand end of its stroke. PII having been positioned as aforesaid, PH is now locked, and the signal for this to take place may be the coincidence of the said indication of the position of PI and a similar indication that PII has been positioned as aforesaid. At the instant when both feet become locked together TH may still be open, but before PI reaches the end of its stroke, VII must be connected to receive the driving signals and TH must be closed. There must be a time when both drives are receiving the driving signals to guard against any gap when neither of them are receiving driving signals, since any such gap would leave table 1 out of control.

If the sense of the driving signals remains the same, the next step is to release Fl, remove the first drive from control by the driving signals by disabling VI, operating. TI, and conditioning the first drive for control by the positioning means. If these functions cannot take place simultaneously they should follow one another in the order stated.

If the driving signal calls for a change of direction of the table immediately after the change-over from the first drive to the second drive has been completed, piston PII will be close to the right hand end of its stroke and it will move closer still to this end of its stroke in the course of driving table 1 in the new direction. This will bring about a rapid change-over to control by the first 'drive again, the sequence of events being similar to those occurring on a change-over from the first drive to the second drive, as previously described.

If the driving signal calls for a change of direction of the table during the short interval of time that both feet are locked and both driving mechanisms are being actuated by the driving signal, then it is necessary to arrange that the first driving means resumes the driving function without completeing the change-over. This can 'be arranged by selecting the foot to be released according to which of the pistons moves towards the centre of its stroke during the interval when both feet are locked. For instance PI, with rightwards travel of table 1, had approached the left hand end of its stroke and PH had been positioned towards the right hand end of its stroke prior to the locking of both feet, in the example quoted above. If now the driving signal causes table 1 to move to the left, PII will approach yet nearer to the right hand end of its stroke and PI will recede from the left hand end of its stroke. This state of affairs must be made to select FII (rather than F1) to be unlocked; freeing from control by the driving signal and conditioning of the driving mechanism for positioning must always be applied to the drive mechanism whose foot is released. Furthermore once afoot is released in this way, these other events should be completed independently of any change of the sense of the driving signal during any period of time which may elapse during the completion of the said events.

A subsidiary advantage of the invention relates to the means for monitoring the motions made good by the table 1. Such means are commonly employed to complete a feedback servo loop wherein command signals representing motions which the table is required to make are continuously compared, with due regard to sign, with the monitoring signals, to produce a difference or error signal which is applied to the actuator of the operative driving mechanism. A commonly used monitoring means comprises a pair of optical diffraction gratings, a long grating and a short grating, one attached to the table and the other attached to the slideway. The rulings of the gratings run transversely of the direction of motion of the table and the two gratings have the directions of their rulings slanted at a small angle to one another. The

gratings are superimposed with a small gap between adjacent flat surfaces thereof and are mounted so that they remain in overlapping relationship and with substantially uniform spacing between the said flat surfaces, in all positions of the table along its slideway. When a light source is viewed through both gratings dark bands (socalled moir fringes) running transversely of the rulings of the gratings are seen. These bands may be scanned through a slit running parallel to the bands, by means of a light cell. When the gratings make relative movements on movement of the table, the bands move transversely of the gratings though at a considerably magnified velocity. Such movement causes a substantially sinusoidal variation of the output of the light cell, one cycle of such variation corresponding to a movement of the table of a distance equal to the distance between adjacent rulings of the grating. Signals in either digital or analogue form representing the actual motion of the work table can readily be derived from the output of the light cell.

A problem arises in connection with monitoring systems of this type in that one of the gratings has. to be somewhat longer than the distance the table is required to travel. This distance may amount to several feet and diffraction gratings of this length are difficult to-make and difficult to mount.

With the arrangement of the invention, each driving mechanism may be supplied with a separate difiraction grating assembly and as the travel of each drive mechanism is relatively short, the use of a long diffraction grating may be avoided. When driving action is changed over from one driving mechanism to the other, the monitoring function is also changed over and the total count of fringes is asaccurately maintained as in the case of a single long diffraction grating assembly provided that steps are taken to ensure that count units are not lost or spuriously added in due to misphasing of the two systems at the instant of change-over.

In the embodiment illustrated in FIGURE 1 the positioning of the unlocked foot is ensured by mechanical means.

An auxiliary hydraulic cylinder C111 is mounted on the underside of table 1. A piston PIII capable of doubleacting operation, rides in G111 and is mounted on a piston rod 3, the ends of which pass through glands in the cylinder ends of CHI and are secured to a depending cradle 4, the lower end of which carries a pivot on which is mounted a pinion 5. This pinion engages two toothed racks 6 and 7 one attached to each of the feet FI and F11. The piston rod extends beyond the right head end of the cradle and enters a valve VIII the body of which is attached to table l and is provided with a bore and ports for controlling the application of hydraulic fluid direction in relation to its cylinder.

with or fastened to the end of the piston rod 3. The valve VIII is so arranged that when PIII moves from the centre of its stroke in CIII the valve operates to restore it to the centre. A tap TIV controls the supply of hydraulic fluid to VIII from the high pressure side of the system. A tap TIII interconnects the two ends of CIII so that PIII can move without constraint when the tap is open. I

When P111 is held in the centre of its stroke in CHI by the action of VIII, any movement by the piston of one of the drives relative to its cylinder, results in a movement of the piston of the other drive in the opposite This can only happen when one of the feet is unlocked. When both feet are locked, in the process of changing over from the drive by one driving mechanism, to drive by the other, the two racks 6 and 7 Will be locked to the bed 2 and the pinion 5 Will also be held fast between them. To enable this to happen, tap TIII must be open to enable PHI to move freely in CIII as the latter moves to the right with the table 1.

Suppose that the table has been moving to the right for instance, driven by PI/CI and, PI having neared the left-hand end of its stroke in CI, both feet have become locked. During the prcvic IS movement to the right of table 1, P111, held in its central position by VIII, will have carried the carriage 4 and the pinion 5 bodily to the right, causing the pinion to rotate anti-clockwise in rolling along rack 6. The combined rotary and bodily movement of the pinion will have driven rack 7 to the right. The bodily movement towards the right, of the pinion 5 balances the rightwards movement of C11 with table 1 and the rotary movement of the pinion 5 increases the rightwards movement of PH so that it exceeds this rightwards movement of CH with table 1, thus causing PII to approach the right-hand end of CH.

Also during this time T II will have been open to free PII from constraint by the hydraulic fiuid in CH and AH will have been disconnected from the driving signals.

Before both feet are locked, TIII must be opened, and during any interval between the two events FII will be freed from constraint by rack 7, and as CII will still be moving to the right with table 1 this, in conjunction with the friction of FII on bed 2, would tend to move piston PII back towards its central position. On the other hand any resistance to flow through TH and TIII tends to hold PII at the right hand end of CH. It is of little moment which of these tendencies predominates because the interval is short.

The locking of both feet and the accompanying operations are initiated when both the pistons PI and P11 have reached corresponding positions near to the opposite ends of their respective strokes. To bring this about each drive is provided with a pair of limit switches, 18 and 28 for PI, CI, FI and 3S and 48 for PII, CIl, FII. These pairs of switches are operated by cams Cal and Call carried by one of the upstanding brackets of F1 and F11 respectively.

FIGURE 2 shows a circuit for controlling the changeover between the two driving mechanisms by means of relays of the telephone type.

Contacts 181 and 281, of switche 13 and 2S respectively, are arranged in parallel so that each can form an operating circuit for a relay P. Contacts 381 and 481, of switches 38 and 45 respectively, are similarly arranged in parallel so that each can form an operating circuit for a relay Q in series with normally open contacts p1 of relay P. Only when one switch of both pairs is closed can Q operate. When Q operates, its normally open contacts q1 cause operation of a double-wound relay R over its operate winding, On operation of R, the closing of its normally open contacts r1 complete a holding circuit, prepared at p2 on operation of P, over the holding winding of R. The operation of Q, followed by the operation of R has the following results.

TI and LVI and TH and LVII each have electromagnetic actuators. These actuators are designated TI, LVI, TIL LVII in FIGURE 2. When the actuators are energised the respective taps T are closed and the foot looking control valves LV are opened to lock the respective feet.

TI and LVI are connected in parallel in a normally made operating circuit over back change-over contacts r2 whilst T11 and LVII are connected in parallel in a normally open operating circuit including front change-over contacts r2. Contacts (13 bridge together these two operating circuits so that both are completed when Q is operated. Q operates first and tap T11 is closed so that PII/CII can drive under control of VII when the driving signals are applied to its actuator AII. Also LVII is operated to lock foot FII. Immediately after the operation of Q, R operates and changes over r2 but this has no elfect on the TI, LVI, TII, LVII operating circuit so long as Q remains operated.

A similar circuit comprising change-over contacts 1'3 and normally open contacts g5 operates to connect the driving signals to the two main actuators AI and All simultaneously, whilst Q remains operated. Preferably g5 contacts are adjusted to close before q3 contacts since it is necessary that the position of VII should be under control of All before the tap T11 is closed since the op eration of PII/CII would be arbitary if TII were to be closed with VII in an arbitrary position and this might resuit in a brief interval of time during which PII/CII was opposing the action of PI/CI.

Taps T111 and TIV are also supplied with electromagnetic actuators designated T111 and TIV in FIGURE 2. These actuators are arranged to be operated one at a time via change-over contacts L14. When Q is unoperated TIII is energised holding tap TIII closed; when Q is operated TIV is energised over q4 front contacts closing tap TIV. Thus when both feet are locked P111 is free to move in its cylinder CIII.

The drives of FIGURE 1 are each equipped with a diffraction grating position monitoring system of the type previously described. Each system has a relatively long diffraction grating carried by a bracket mounted on the top of one of the upstanding arms of the foot and a cooperating small diffraction grating together with light source, slit, and photoelectric cell mounted on the table. The light cell for F1 is designated MI and the light cell for P11 is designated MII. It is necessary to change-over the connections to the comparator, in which the monitoring signals are compared with the command signals, from one monitor system to the other when the drive is changed from one drive to the other.

This change-over may be done by electronic means which make the change instantaneously. As the signals from the two monitoring signals will only rarely be in phase when both feet are locked, there will be an error if the changeover takes place with no previous phasing operation but if the unit of distance represented by a cycle of the monitoring means is made small in relation to the limitsof accuracy to which the motion of the table is to be controlled, this misphasing error can be ignored. Over a large number of successive changes of driving action the errors will in any event tend to cancel one another on a statistical basis.

It is nevertheless preferable to arrange to bring the monitoring signals into phase before changing over and this is preferably undertaken by an electronic phasing circuit illustrated schematically in FIGURE 2 by the cir cuit denoted Monitor phasing circuit. This circuit is energised over (16 contacts on operation of Q and being of the electronic type it can be made to respond before the locking of the hitherto free foot becomes eifective. Many methods are available for bringing the two monitor systems into phase.

One such method is to vary the relative positions of the two feet immediately prior to the locking of the hitherto free foot. This may be done by a suitable shift of the position of piston PIII from its central position for instance by means of an electromechanical transducer inserted between piston rod 3 and the valve spool of VIII, a phase correction signal from the Monitor phasing circuit being applied to the transducer so as, in efiect, to lengthen or shorten piston rod 3. For this method to be effective the locking action of the hitherto free LV lock valve must be synchronised with the opening of tap TIII so that there is no period when the foot is out of control and free to misphase the two monitors again.

When the two feet have been locked, with the monitors in phase, the change-over between the two monitors can take place at any convenient instant so long as there is no gap when no monitor is connected to the servo signal comparator.

The change-over is effected by change-over contacts rdof relay R and to bridge the gap when the blade of r4 is crossing the back and front contacts, both monitors MI and M11 are bridged together by a temporary connection made under control of the Monitor phasing circuit, illustrated schematically in dotted lines as a relay contact set mp. When both feet are locked and the driving signals are being applied to both actuators AI and All, if the driving signals do not change their sense, so that in the example under discussion, they still call for right- Wards movement of table 1, then PI will move closer still to the left hand end of its stroke and switch contacts 251 will remain closed. PII on the other hand, being positioned at the right hand end of its stroke will move to the left in relation to C11 and switch contacts 351 will open. Q will release but R will hold over r1 and p2; also r2, r3 and r4 will remain operated holding FII locked by means of LVII, TII closed, AII connected to receive the driving signals and M11 connected to the servo signal comparator whilst the release of Q unlocks FI by means of LVI, opens TI, disconnects AI from the driving signals, and- MI from the servo signal comparator and on the release of g4 PIII/CIII will be restored to normal operation by opening TIV and closing TIII, whereupon FII will start to move to the right under control of racks 6 and 7 and pinion 5. This will soon cause switch contacts 281 to open, releasing P but R will still hold over back contacts q2 in parallel with p2, despite the opening of p2. If there is no change in the direction of the driving signals the second drive will continue to drive table 1 to the right with relay R operated, and the first drive will be positioned so that PI moves towards the right hand end of C1 whilst PII moves towards the left hand end of C11. On continued rightwards movement of table 1 switch contacts 451 and 151 will eventually close and operate P and Q again, R continuing to hold over p2 on the opening of q2. The phasing of the 7 monitors will be effected and both feet will be locked as described above in relation to the change-over from the first drive to the second drive. If the rightwards movement of table 1 continues switch contacts 481 will remain closed and switch contacts 181 will open to release P. R hold circuit will then be broken at p2 and R will release. Q also will be released at p1. To prevent the release of R being prejudiced'by the premature closure of q2 back contacts, Q is made slow-to-release. All the relays will now be released and the table 1 will be driven by the first drive, the second drive being positioned with its foot FII unlocked. The relay contacts will then all be in the states shown in FIGURE 2. If, however, in the course of the change-over from the first drive to the second drive during the period when both feet are locked, the driving signals call for a change of direction of table 1, PI will reverse direction towards the centre of its stroke and switch contacts 281 will open leaving switch contacts 351 closed. This will release P, and p1 will open to release Q and p2 will open to release R. The circuits of FIGURE 2 will now all be restored to the state shown in the figure and the first drive will resume the driving function.

When table 1 is moving to the left, the change-over is initiated by the closing of switch contacts 181 and 481 but the sequence of operations of the P, Q, and R relays is as above described for the case where the change-over is initiated by switch contacts 281 and 351.

FIGURE 3 shows the foot locking arrangement in cross section on an enlarged scale.

The bed on which the foot slides when free, is T- shaped and the lower surfaces of the projecting flanges of the bed are engaged by flanges inwardly projecting from independent side walls of the foot which embrace the flanges of the bed. The foot F is spaced from the vof the hydraulic pressure supply system whereupon plunger L is'forced into engagement with the bed to lock foot F to the bed. Alternatively valve LV can be operated to connect LC to the lowpressure or exhaust side of the hydraulic pressure system whereupon plunger L is released from engagement with the bed and foot F is free to slide on the bed.

FIGURE 4 shows an embodiment of the invention which is a modification of the embdiment illustrated in FIGURE 1 in that only one main hydraulic control valve is required. The control circuits of FIGURE 2 may be used with the omission of the portion FIGURE 2C.

The actuator AI for this single control valve is permanently connected to the servo signal generator and the valve is connected to the two cylinders CI and CH by ducts which effectively place the two cylinders in series. One of these ducts, 8, connects one outlet from the valve VI to the right hand side of CH and another of the ducts, 9, connects the other outlet from the valve VI with the left hand side of CI. The third of the ducts, 10, connects the right hand side of CI to the left hand side of CH.

When TI is open the ducts 9 and 10 are connected together and the pressures on the two sides of P1 are equalised so that cylinder CI is idle and no opposition is offered to the free movement of piston PI therein. Under these conditions the valve VI controls the drive CII, PII and CI, PI is free to be positioned. When, conversely, TI is closed and TII is open cylinder C11 is bypassed and ducts 3 and it are connected together to connect the valve V I to the right hand side of CI.

When in the course of a change-over operation, both feet FI and FII are locked and both taps TI and TII are closed the two pistons PI and PH become virtually one piston and hydraulic fluid displaced from the one the piston of which is moving towards the end of its cylinder communicating with duct it will pass into the end of the other cylinder from which the corresponding piston is retreating. This arrangement avoids hydraulic transients at the instant of change-over from one drive to another because there is no pressure difference across the piston, nor across the tap, of the idling cylinder so long as the tap is open and the locking of the foot of this drive causes the piston to move in the same direction as it will be urged to move by the hydraulic fluid passed to it from VI when its tap is closed.

In other respects the operation of the arrangement shown in FIGURE 4 is the same as that previously described in relation to FIGURES l, 2 and 3.

In an arrangement similar to that illustrated in FIG- URE l, the positioning of the free foot may be controlled electrically rather than by mechanical means such as the racks 6, 7, pinion 5, cradle 4, and piston cylinder arrangement PIII/ CHI.

This can be achieved by providing auxiliary hydraulic piston and cylinder assemblies connected in parallel with the main driving pistons and cylinders PI/ CI and PII/CII.

The energisation of these auxiliary cylinders may be controlled by auxiliary switches similar to 18 and 25 (or 33 and 48) but set closer together so that one or other of these is operated when the driving piston has moved but a short way in one direction or the other from its central position. The action initiated by this auxiliary switch is such as to drive the idling drive towards the appropriate end of its stroke until one of the limit switches 18/28, or 38/48 is operated whereupon, if and when the active drive has also operated one of its limit switches the change-over action proceeds under control of circuits such as those illustrated in FIGURE 2. If the driving signals reverse the direction of the active drive before such a changeover is initiated, then the ultimate release of the auxiliary switch previously operated by the active drive is arranged to cutoff the positioning action of the auxiliary cylinder of the idling drive so that it no longer approaches a limit switch. If the driving signals retain this changed direction the other auxiliary switch will be operated and the idling drive will be positioned in the opposite direction by its auxiliary cylinder until the other limit switch is operated. Preferably the positioning action of the auxiliary cylinder is cut off when the drive which is being positioned operates one of its limit switches. The drive will then retain the position so attained until the active drive operates one of its limit switches, to initiate the change-over as previously described.

Another method of positioning the free foot in an arrangement similar to that shown in FIGURE 1, is to apply the servo or command signals in reversed sense to the valve controlling the corresponding piston/cylinder assembly. The arrangement shown in FIGURE 1 is modified by the omission of the rack and pinion assembly 4, 5, 6, 7 and of the piston, cylinder, valve and tap assembly PHI, CIII, VIII, Till and TIV. The control circuits also require to be modified by the substitution of the circuits shown in FIGURE for the B, C, D and E portions of FIGURE 2.

In FIGURE 5, two relays U and W, with associated relays UA, UB, UC, and WA, WB, WC respectively are responsible for the control of the PI/CI and P-II/CII piston/ cylinder assemblies also respectively. a

Except during a change-over operation, one of the pair of relays U and W is operated and the other released. Each of these relays has two operate paths, one via contacts 1'2 and the other via back contacts of one of the associated relays of the other of the pair. The operation of R is associated with drive by PII/Cil and when these conditions obtain r2 front contacts are closed to provide an operate path for relay W and the r2 back contacts are open to deny the corresponding operate path to relay U. The second operate path for U is broken at the wcl contacts of WC one of the relays associated with W. Operation of W operates WA over W2. WA operates WB over wa2. WB operates WC over wb2. Similarly, non-operation of R is associated with drive by PI/CI and when these conditions obtain r2 back con-tacts are closed to provide an operate path for relay U and r2 front contacts are open to deny the corresponding operate path to W whose other operate path is broken at ucl, the relays UA, UB and UC being operated on operation of U in the same way as are WA, WB and WC on operation of W.

UA, U3 and UC being operated hold Tl operated (tap closed) over uc2. LVI operated (foot locked) over ual and VAI connected to the source of command signals at ubl.

During the course of driving operations by PI/CI positioning of foot FII has been proceeding my means of the application of command signals of reversed sign to the actuator VAII of valve VII. These signals are preferably amplified to ensure that foot P11 is positioned suitably some time before PI-I/CH will be required to take over the driving function. When the circuit of FIG. 5 is used each of the limit switches has two contact sets, one normally open (eg. 181 in FIGURE 2A) and one normally closed (cg. 182, FIGURE 5) and as soon as FII has been positioned to the point at which one of the limit switches 38 or 48 is operated, either 332 or 4-82 switch contacts open to disconnect VAII from the source of amplified reversed command signals. This must have the effect of leaving PII/CII neutralised which may be efiected by' spring loading valve VII to a neutral position to which it will automatically return when VAII is disconnected.

If, now, a change-over operation is initiated, the circuits of FIGURE 2A cause the operation of P, Q, and R (in that order), as previously described in relation to the circuits of FIGURE 2, A, B, C, D and E. Contacts q3 (FIGURE 5 join W to the operate circuit of U and when r2 contacts change over an instant later there is no break in this circuit since r2 contact set is of the make-beforebreak type.

W will now operate with the following results: WA is slow to operate and operates after an interval over W2; wa1 operates LVII to lock foot =FII; wa2 operates slowto-operate relay WB after a time lag; wbl transfers VAII (already disconnected from the reversed command signals at 382 or 482) to the command signal source-- both drives are now working together; wb2 operates slowto-operate relay WC; wcl breaks the operate circuit of U but U is still held over r2 front contacts and (13 front contacts so that U does not release.

Ifmovernent of the slide in response to the command signals continues in the same direction PII/CII will retreat from the limit switch (33 or 48) to which it was previously positioned and either 381 or 451 will open to release Q with R remaining operated over r1 and p2 in the,

first instance and later over r1 and q2 (back), the latter contacts replacing p2 when P releases on the opening of 181 or 182 soon after the commencement of the positioning of PI/ CI. In FIGURE 5, W is held operated over r2 front contacts but U is released on the opening of q3 since its alternative operate path is broken at wcl (WC being operated as previously indicated).

The release of U has the following results: ul back contacts close an alternative holding circuit for TI so that TI is not opened on the subsequent release of UC: uZ releases slow-to-release relay UA; ual releases LVI to unlock foot FI; uaz releases slow-to-release relay UB'; ubl back contacts prepare a circuit from the source of reversed command signals to VAI. This path is broken at one of the contacts 182 or 282 and the operated switch will not be released until the first drive moves off the operated limit switch under the influence of the reversed command signal. To make this possible each of the contacts 182, 282, 382 and 452 is bridged by a resistance of such a value that a much attenuated reversed command signal can pass when the contacts are open. The first drive can then commence a very slow positioning action until the opened contacts (182 or 282 as the case may be) closes to apply the full strength of the amplified reversed command signal to VAI which causes P I/CI to be positioned until the other limit switch is operated whereupon the reversed command signals applied to VAI are again attenuated. The amplification factor and the value of the resistance are chosen so that the attenuated signal is not suflicient to damage the piston and cylinder when it reaches the end of its stroke. Indeed a' careful choice of these values in relation to the maximum speed of the table 1 can ensure that the drive which is being positioned can never reach the end of its stroke; ub2 releases UC; ucl restores the alternative operate circuit for W; uc2 disconnects one of the operate paths of TI but the latter remains held over M1.

It will be noted that TI is only opened for a brief interval between the operation of U (breaking ul) and the operation of UC (closing uc2) that is to say during the successive operate lags of UA, UB and UC. The same applies to TII mutatis mutandis. During driving and positioning the taps must be closed and, with this method of positioning the idle drive, it is only necessary to provide the taps so that the piston of the erstwhile idling drive is not locked in its cylinder at the instant when its foot it locked by operation of UA or WA as the case may be, and before the correspondingcontrol valve has been switched to control by the commond signals on operation of UB or WB as the case may be.

With the arrangement of FIGURE 5 the monitoring system control circuits may be arranged as in FIGURE 2E, in which case the signals controlling the valve actuator of the working drive will be the resultant of incoming programme signals and monitoring signals from the monitoring system of that drive and the reversed signals for positioning the foot of the idle drive will be derived from those resultant signals. It is possible, however, to keep the monitoring system of the idle drive in operation by providing individual comparator circuits for the two drives and applying the reversed signals for the idle drive to its individual comparator the output of which is passed to the valve actuator of the idle drive to control positionl l ing of its foot. This ensures that the positioning operation proceeds at a speed governed by the setting of the amplifier inserted in the reversed signal circuits without regard to the friction or other loads resisting the positioning of the idling drive.

In either event, of course, there remains the necessity of phasing the two monitor systems during a change-over from one drive to another. This phasing problem may be solved by locking the feet only at predetermined positions along the length of the bed; for instance by means of a tapered pin engaging any one of a number of accurately located holes along the length of the bed. FIGURE 6 shows in detail an arrangement of this type. A foot F, drawn to an enlarged scale as compared with FIGURE 1, is provided with a number of cylindrical bores LC (such as LC in FIGURE 3), spaced apart along the direction of movement of the foot when idle. In each of the bores LC is a plunger L (corresponding to plunger L in FIGURE 3). The plungers have an upper cylindrical portion acting as a piston, an intermediate accurately ground tapered portion and a lower rounded tip. Spaced along the bed is a series of holes LH tapered to engage accurately the tapered intermediate portions of the plungers L. The pitch of the holes LH differs from that of the plungers L and in FIGURE 5 the former is smaller than the latter. A number of plungers L are provided to enable the spacing apart of discreet positions in which a foot can be locked to be kept small without the necessity of having an inordinately large number of closely spaced holes along the bed. The difference in pitch between plungers L and holes LH ensures that one of the former will be close to a hole at the instant when locking isrequired and the rounded tip of that plunger will cause it to pull the foot along till the plunger in question is lined up with the nearest hole which it will enter to locate the foot accurately by engagement of its intermediate tapered section in the tapered bore of the hole.

The plungers L are all connected to the same hylraulic fluid gallery LG and each is urged away from the bed by a tension spring. The plungers other than the one which is near enough to a hole to enter it, will be forced down upon the top surface of the bed When the lock valve LV operates but the pressure need not be great as the locking of the foot is ensured by the direct mechanical engagement of a plunger in one of the holes. Thus the pressure of the tips of the said other plungers upon the surface of the bed will not be such as to resist the pulling-in action of the plunger which engages a hole to any significant extent.

With this arrangement the provision of the taps TI and T11 and the circuits a1, 1102 and W1, wcZ which open the taps only during the time when a foot is actually being locked, are essential in order that the operative one of the plungers L may be free to draw the foot to the nearest discreet locking position without any resistance from the piston and cylinder of the drive in question. With the locking arrangement of FIGURE 3 however, it is possible to omit the taps TI and T11 provided that somewhat modified control circuits are used.

FIGURE 7 shows such circuits. In FIGURE 7 relays and their contacts having corresponding functions are given the same designations as in FIGURE 5.

Suppose that PII/CII is driving: R is operated holding W operated at r2 front contacts; WA is operated at W1; W2 and w3 front contacts switch WM and WN to the operate paths via wal and wbl respectively; WA holds WB operated at wa2 and WM operated at wal via w2 front contacts; wbl holds WN operated over w3 front contacts; WM, operated, connects the command signal source to VAII at wml; WN, operated, holds foot FII locked by means of LVII energised over wnl, and W112 holds WC operated; wcl breaks the alternative operate circuit of U which is unoperated because r2 back contacts are also open; PI/CI is positioned and has operated one of its limit switches 18 or 28 holding P operated over 181 aosasve or 281; VAl, previously connected to the reversed command signal source over uml back contacts had this path attenuated at 182 or 282 :by the resistance bridging the opened contact, on completion of the positioning operation. PII/CII now approaches the end of its stroke and operates one of its limit switches 38 or 45: 381 or 481 operates Q; 3 operates U over r2 front and g3 front contacts; ul operates UA, n2 and u3 switch UM and UN to operating circuits via ual and ubl respectively; UA, operated, operates, in delayed succession, UM over ual and U3 over M2; UN therefore operates over ub2 in similarly delayed succession after UM; uml front contacts connect the command signal source to, VAI and unl front contacts operate LVI to lock foot FI, these actions following one another in timed sequence; the operationof UN'also operates UC at 1:122 and ac]. back contacts open to break the alternative operate circuit of W but W still holds over r2 front contacts. Both drives are now operating under control of the command signal and PI/CI will be moving away from its operated limit switch 18 or 28 which, on the opening of 181 or 281 contacts, as the case may be, results in the release of P followed by the release of Q and R. This causes W to be released (r2, and q2 front contacts both being now open).

On the release of W, the following events take place: WA is released by the breaking of WI, whereupon (after the release lag of WA) wal releases WN (Whose operate path was changed over from that via wbl on the release of W2; waZ releases WB, whereupon, (after the release lag .of WB) wbl releases WM (whose operate path was changed over from that via wal on the release of W3). It will be seen that WN and WM release in delayed succession in that order. By comparison with the description above the operation of the corresponding relays UM and UN, it will be seen that the order of release of these relays is the opposite of their order of operation. The result of the release of these relays in that order is that the foot of the drive about to relinquish the driving function is released and then, in timed succession thereafter, this drive is removed (at wml) from the control of the command signals and a path prepared for positioning control by the reversed command signals. This path is completed via one of the contacts 382 or 452 which is closed and via the resistance bridging the other of them which is open. The reversed command signals will therefore be applied in attenuated form to VAII and PII/CII Will start the positioning process at slow speed until that one of the contacts 382 or 482 which is opened, is closed on the second drive moving off the operated limit switch, whereupon the reversed command signals are applied in amplified form and the positioning process is accelerated. When the second drive is fully positioned one of the limit switches 38 or 48 again operates to attenuate the reversed command signals again. The idle drive will then move I slowly towards the end of its stroke, over running the operated limit switch. As already indicated in relation to FIGUE 6, it is of no consequence if this decelerated position action causes the idle drive to reach the end of its stroke but this may be avoided by suitable choice of the amplification factor applied to the reversed command signals, the switch contact bridging resistances, the release lags of the various relays culminating in the release of WM (or UM when PI/CI is idling). The valves used for V1 and VII must be of the type such that a given setting produces a given piston velocity, independent of the load on the drive controlled by the valve, since this velocity must not vary on the occurrence of variations in say the cutting load (where the invention is applied to a machine tool). Therefore the magniture of the reversed command signals will similarly govern the positioning velocity. The positioning action of one drive commences after the commencement of the driving action of the other drive since it is delayed during the successive release of W, WA, WB and WM (or U, UA, UB and I UM). It will therefore be required to catch up the active drive. It starts slowly; the attenuated reversed signals may be of the order of .5 of the amplitude of the command signals. When the limit switch is released the positioning action must be accelerated to enable positioning to be completed before the active drive reaches the end of its stroke. The amplified reversed command signals, when no longer attenuated, may then have an amplitude of the order of 1.2 times the amplitude of the command signals. If during this period the command signals reverse direction the reversed command signals will do likewise and the positioning direction will be reversed. The positions of the limit switches also effects the positioning operation but the principal factor governing the placing of the limit switches is the necessity to ensure that the drive about to relinquish the driving function does not reach the end of its stroke during the change-over operation in the brief period when both drives are working together with both feet locked, under control of the command signal.

It is understood that the use of separate short range monitoring assemblies for the two driving mechanisms is not fundamental to the invention. There are methods available for avoiding the use of diifraction gratings or equivalent position monitoring devices having extremely fine rulings or equivalent registration marks whereby the provision of a device as long as the range of movement between the two objects, presents less difiiculty. Such devices may be used in conjunction with the present invention, enabling the phasing arrangements hereinbefore described in relation to FIG. 2E to be dispensed with.

We claim:

1. Driving apparatus for causing relative movement along a predetermined path between a first object and a second object, comprising a first drive and a second drive, said drives being driving mechanisms and each said drive in turn comprising two members between which relative movement takes place to provide the driving action of said drive, the first of said members being secured to said first object and the second of said members being provided with locking means, said locking means being means for releasably securing said second member to said second object at various positions along a track parallel to said predetermined path, actuating means, said actuating means adapted in response to an incoming driving signal to initiate driving action on the part of said drives, positioning means, said positioning means being adapted for positioning said first member of each said drive relative to said second member of each said drive when the drive in question is not executing a driving action, and indicating means, said indicating means being adapted for indicating the relative positions of said two members of each said drive at least at a plurality of predetermined relative positions of said two members, the apparatus further comprising means for operating said locking means of said first drive and for releasing said locking means of said second drive, means for applying driving signals to said actuating means of said first drive, means for actuating said positioning means of said second drive to place members of said second drive in predetermined relative positions, means under control of said indicating means of said first drive for operating said locking means of said second drive and bringing said actuating means of said second drive under control of said driving signals and means, operable when said locking means of both said drives are operated and before either of said drives has reached an end of its range of driving action, for releasing said locking means of that one of the two said drives which is nearest to an end of its driving range when it is at that instant approaching in the course of its driving action and removing said actuating means of that said drive from the control of said driving signals, said first drive and said second drive being interchangeable in respect of the functions of the various means further comprised in said apparatus as aforesaid whereby the two said drives may operate in turn to produce relative movement between said first object and said second object.

2. Apparatus as claimed in claim 1 in which said means for actuating said positioning means of said second drive is under control of said indicating means of both of said drives.

3. Apparatus as claimed in claim 1 in which said positioning means of each of said drives comprises a releasable mechanical coupling between the two said drives arranged so that relative movement in one direction between the two said members of that one of said drives which is for the time being executing a driving action, gives rise to relative movement in the opposite direction between the two said members of that one of said drives which is for the time being not executing a driving action.

4. Apparatus as claimed in claim 3 in which said mechanical coupling acting as said positioning means comprises two racks only secured to said second member of each of said drives and extending inwardly therefrom in overlapping spaced relation, a pinion engaging each rack and supported between said racks upon an axle carried by one member of an hydraulic piston and cylinder assembly the other member of which is secured to said first object, said two members of said hydraulic piston and cylinder assembly carrying respectively two parts of a hydraulic valve controlling the flow of hydraulic fluid to said piston and cylinder assembly, said valve being arranged so as to maintain said piston and said cylinder of said assembly in predetermined relative positions, means for removing said assembly from control by said valve and rendering said piston and said cylinder free to make unhindered relative movements whereby said positioning means may be disabled.

5. Apparatus as claimed in claim 1 in which said positioning means of each of said drives comprises means for applying to said actuating means a signal corresponding to said incoming signal but of reversed sense so that while said incoming driving signal is operating said actuating means of one of said drives to cause relative movement between said two members of said drive in one direction to provide a driving action, said corresponding signal of reverse sense operates said actuating means of the other said drive to cause relative movement between its two said members in the opposite direction.

6. Apparatus as claimed in claim 5 in which said corresponding signal of reverse sense is amplified to accelerate the positioning of said drive which is not executing a driving action as compared with the velocity of relative movement between the two said members of the other said drive which is executing a driving action.

7. Apparatus as claimed in claim 6 with means under control of said indicating means of the said drive which is being positioned for arresting the positioning action when the two said members of said drive have reached predetermined relative positions.

8. .Apparatus as claimed in claim 6 with means under control of said indicating means of a said drive which is being positioned for severely attenuating said corresponding signal of reverse sense when the two said members of said drive have reached predetermined relative positions.

9. Apparatus as claimed in claim 1 in which the various means further comprised in said apparatus are arranged, when said indicating means of both said drives have operated to produce a sequence of events in the following order, first, said positioning means of said second drive is disabled whereby the two said members of said second drive are free to execute unimpeded relative movements, secondly, said locking means of said second drive is operated to secure said second member of said second drive to said second object, thirdly, said actuating means of said second drive is brought under control of said driving signals so that both said drives are executing mutually assisting driving action, fourthly, said 15 locking means of that one of said two drives which is nearest to the end of its driving range to which it is at that instant approaching is released, fifthly, said actuating means of said drive last above referred to is removed from control by said driving signals, and sixth, said positioning means of said drive last above referred to is operated to cause relative movement of said two members of that drive in the direction of opposite end of their driving range.

10. Apparatus as claimed in claim 1 in which there is associated with each of said drives a motion monitoring device so arranged as to monitor relative movements between said first member and said second member of said drive, the apparatus comprising a feedback servo control system adapted to compare an incoming command signal with a signal from said monitoring device associated with that one of said drives which is executing a driving action and from such comparison to derive a difference signal, such difierence signal constituting said driving signal to which said actuating means of that said drive is responsive.

11. Apparatus as claimed in claim 10 with means for changing over from one of said monitoring devices to the other for comparison of its outward signal with the incoming command signal, such changeover taking place at a time when said locking means of both said drives are operated to secure said second members of both said drives to said second objects.

12. Apparatus as claimed in claim 10 with means for bringing said signals of both said monitoring devices into synchronism before changing over from one to the other as aforesaid.

13. Apparatus as claimed in claim 11 in which said locking means of each said drive comprises at least one detent coupled to said second member of said drive'and adapted to be moved into or out of engagement with any one of a series of depressions at predetermined positions along said track of said second object.

14. Apparatus as claimed in claim 13 in which said depressions are equally spaced along said track and in which each said locking means comprises a plurality of detents spaced apart along the direction of said track with a spacing which dilfers from the spacing between adjacent depressions in said track. 7

15. Apparatus as claimed in claim 1 in which each of said drives takes the form of an hydraulic piston and cylinder assembly, in which said actuating means is an electro mechanical transducer arranged to operate an hydraulic valve controlling the supply of pressurized hydraulic fluid to said piston and cylinder assembly.

16. Apparatus as claimed in claim 15 in which each hydraulic piston and cylinder arrangement is provided with a by-pass tap which, when opened, permits relative movement between said piston and said cylinder unhindered by the presence of hydraulic fluid within said cylinder. 17. Apparatus as claimed in claim 16 in which a single valve controlled by a single electro mechanical transducer controls the two said piston and cylinder assemblies, said piston and cylinder assembly being connected in series in the hydraulic flow circuit, each piston and cylinder assembly being removable from control of said single valve by the opening of its associated said bypass tap.,

References Cited in the file of this patent UNITED STATES PATENTS 2,311,142 Turretini Feb. 16, 1943 2,679,177 Gepfert May 25, 1954 2,694,804 Wagner Nov. 16, 1954 FOREIGN PATENTS 409,622 Italy Feb. 23, 1945 

